Build out router-welcomer

* Develop `router-welcomer` fully as a
  * pipelined impl
  * static definitions for all handlers
  * emits `welcome` messages along with sending reply
  * emits both client and server `abort`
* Rebuild `client-helloer` in a similar style
* Implement `pipeline2` - like pipeline, but with context
* Common `abort-snoop` for spying on egress